Radiotherapy is an important treatment modality for prostate cancer. The technological advances in real-time ultrasound image guidance for High-Dose-Rate (HDR) prostate brachytherapy have placed this treatment modality at the forefront of innovation in the field of cancer radiotherapy. HDR prostate brachytherapy generally involves using a radiation source (Iridium-192) to deliver high radiation dose to the prostate gland through a series of applicators that are temporarily placed within the prostate transperineally under the transrectal ultrasound guidance. The dose decreases exponentially with increasing distance from the radioactive source according to the inverse-square law. This procedure allows the dose delivered to the surrounding normal tissues to be minimized.
Recent data shows an improved efficacy of this treatment approach in patients with locally advanced cancer when compared with conventional external beam radiotherapy. As a result, an increasing number of men, many of younger ages, have been undergoing prostate HDR brachytherapy instead of radical prostatectomy for localized prostate cancer.
One key to the success of the HDR prostate brachytherapy is the accurate localization of the prostate, for example, in the treatment-planning images. If the prostate is not accurately localized, a high therapeutic radiation dose could be delivered to the surrounding normal tissues (e.g. rectum and bladder) during the treatment. This may cause severe complications such as rectum bleeding and lead to an under-treatment of the cancerous regions within the prostate gland; therefore result in poor treatment outcome.
Treatment-planning images are often computer tomography (CT) images. However, CT images can have poor soft-tissue contrast and thus lead to inaccurate prostate localization and inaccurate treatment planning. The accuracy and reproducibility of prostate volume manually contoured on CT images among physicians has been found to be poor. It has been shown that CT-based prostate contours consistently overestimate the prostate volume by 30% to 50%.